The Cell (Plasma) Membrane
The cell is enclosed in a thin membrane that separates the intracellular contents from the extracellular environment. To differentiate it from the other cell membranes, such as the mitochondrial or nuclear membranes, the cell membrane is often called the plasma membrane. In many respects, the plasma membrane is one of the most important parts of the cell. It acts as a semipermeable structure that separates the intracellular and extracellular environments. It provides receptors for hormones and other biologically active substances, participates in the electrical events that occur in nerve and muscle cells, and aids in the regulation of cell growth and proliferation.
The cell membrane is a dynamic and fluid structure consisting of an organized arrangement of lipids, carbohydrates, and proteins (Fig. 4.8). A main structural component of the membrane is its lipid bilayer. It is a bimolecular layer that consists primarily of phospholipids, with glycolipids and cholesterol. This lipid bilayer provides the basic fluid structure of the membrane and serves as a relatively impermeable barrier to all but lipid-soluble substances. Approximately 75% of the lipids are phospholipids, each with a hydrophilic (water-soluble) head and a hydrophobic (water-insoluble) tail.9 Phospholipid molecules along with the glycolipids are aligned such that their hydrophilic heads face outward on each side of the membrane and their hydrophobic tails project toward the middle of the membrane. The hydrophilic heads retain water and help cells stick to each other. At normal body temperature, the viscosity of the lipid component of the membrane is equivalent to that of olive oil. The presence of cholesterol stiffens the membrane.
Although the lipid bilayer provides the basic structure of the cell membrane, proteins carry out most of the specific functions. The integral proteins span the entire lipid bilayer and are essentially part of the membrane. Because most of the integral proteins pass directly through the membrane, they are also referred to as transmembrane proteins. A second type of protein, the peripheral proteins, is bound to one or the other side of the membrane and does not pass into the lipid bilayer. Removal of peripheral proteins from the membrane surface usually causes damage to the membrane.
The manner in which proteins are associated with the cell membrane often determines their function. Thus, peripheral proteins are associated with functions involving the inner or outer side of the membrane where they are found. Several peripheral proteins serve as receptors or are involved in intra- cellular signaling systems. By contrast, only the transmembrane proteins can function on both sides of the membrane or transport molecules across it.
Many integral transmembrane proteins form the ion channels found on the cell surface. These channel proteins have a complex morphology and are selective with respect to the substances they transmit. Mutations in these channel proteins, often called channelopathies, are responsible for a host of genetic disorders. For example, in cystic fibrosis, the primary defect resides in an abnormal chloride channel, which results in increased sodium and water reabsorption that causes respiratory tract secretions to thicken and occlude the airways. A recent discovery showed there are specific water channels or pores called aquaporins in the plasma membrane. It is now known that aquaporin disorders are responsible for a number of diseases, including nephrogenic diabetes insipidus.
A fuzzy-looking layer surrounding the cell surface is called the cell coat, or glycocalyx. The structure of the glycocalyx consists of long, complex carbohydrate chains attached to protein molecules that penetrate the outside portion of the membrane (i.e., glycoproteins); outward-facing membrane lipids (i.e., glycolipids); and carbohydrate-binding proteins called lectins. These proteins [lectins] are responsible for a variety of activities and have antitumor, immunomodulatory, antifungal, and HIV-1 reverse transcriptase inhibitory processes. The cell coat participates in cell-to-cell recognition and adhesion. It contains tissue transplant antigens that label cells as self or nonself. The cell coat of a red blood cell contains the ABO blood group antigens. An intimate relationship exists between the cell membrane and the cell coat. If the cell coat is enzymatically removed, the cell remains viable and can generate a new cell coat, but damage to the cell membrane usually results in cell death.